1. Field of the Invention
The present invention relates to a method of determining a timing at which an image of an illuminated surface area is read in for image processing during surface defect inspection.
2. Description of Related Art
In typical automobile manufacturing lines including painting lines, a paint film defect inspection station, at which scratches, projections, cracks, dimples, smears and the like are detected, is located at the end of, or otherwise after, the painting line. A painted vehicle body is transferred by means of, for instance, a chain conveyor at a specified speed to the paint film defect inspection station. An image processing type of surface inspection device is used in the paint film defect inspection station to detect paint film defects. For a detailed description of a representative example of such an "image processing type" of surface inspection device, reference may be had to, for instance, Japanese Unexamined Patent Publication No. 62-233710. With the image processing type surface inspection device described in the above-mentioned publication, a specified area of illumination scans the surface of a vehicle body and an image pick-up device provides images of each scanned area.
Because scanning the surface of a vehicle body with an area illumination divides the surface into a plurality of surface areas according to scanning intervals, the entire surface is imaged by picking up images, one after another, at a timing determined based on the scanning interval. As shown in FIGS. 11A and 11B, a monochromatic image of a surface area is transformed into two-valued image. In the two-valued image, an illuminated surface area appears as a strip image expressed by a single group of bright picture elements in a dark background. Histograms of bright picture elements are produced for each X coordinate which is perpendicular to an image scanning direction Y. A histogram larger than a predetermined threshold value is detected as a scanning interval. The timing of image reading is determined based on the scanning interval.
Even though a specified area of illumination is given, if changes in an incident angle, at which light strikes a subject surface of inspection, are caused, then the width of an illuminated area changes. Such a change occurs due to unevenness of a subject surface. In such a case, the interval of image reading changes according to unevenness of the subject surface. For instance, the interval of image reading is smaller for a surface area containing a difference in level than for an even surface area. For this reason, the prior art surface inspection, in which the image reading interval is determined based only on a scanning interval, causes omissions of inspection between adjacent illumination areas.